How biofilms form

In just a few days or weeks, a microbial ecosystem will form on a clean sterile glass slide placed in a stream of water containing even minimal nutrients. This ecosystem will consist of a variety of microorganisms embedded in a mass of extracellular polysaccharides of their own making. The formation of this biofilm is not a random process but can actually be predicted and recorded as illustrated below. 


© Wiencek, author. ASM MicrobeLibrary

Within minutes an organic monolayer can adsorb to the slide surface, changing the chemical and physical properties of the glass slide. These organic compounds are made up of polysaccharides or glycoproteins which condition the surface of the slide and increase the probability that planktonic bacteria will attach. Free-floating or planktonic bacteria encounter the conditioned surface and form a reversible, sometimes transient attachment often within minutes.

This attachment or adsorption is influenced by the electrical charge of the bacteria, by Van der Waals forces and by electrostatic attraction. The precise nature of the interaction is still a matter of intense debate. In some instances, such as in the association between a pathogen and the host cell receptor sites, there may be a stereospecificity which is stronger than ionic or electrostatic forces.  At this point the stereospecificity is still reversible.

However, if the association between the bacterium and its substratum persists long enough, other types of chemical and physical structures may occur which transform the reversible adsorption to a permanent and essentially irreversible attachment.

The final stage in the irreversible adhesion of a cell to an environmental surface is associated with the production of extracellular polymer substances or EPS. Most biofilm EPS contains sugars such as glucose, galactose, mannose, fructose, rhamnose, N-acetylglucosamine and others.

This slime layer of EPS and bacteria entraps particulate materials such as clay, organic materials, dead cells and precipitated minerals, adding to the bulk and diversity of the biofilm habitat. The biological diversity of the community may continue to increase as the biofilm attracts the attachment and growth of other organisms.

Introduction
Where biofilms are found
How biofilms form
Biofilm attributes
Facts about biofilms
A misconception

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